1. Field of the Invention
The invention relates to a bearing apparatus including a bearing portion and a lubrication unit that allows the bearing portion to be lubricated, and a lubrication unit provided in a rotating apparatus.
2. Description of the Related Art
In recent years, for a variety of machine tools, there has been a demand to allow a main spindle to operate at a higher speed in order to increase machining efficiency and productivity. When the main spindle rotates at high speed, particularly lubricity for a rolling bearing that supports the main spindle becomes a problem. Oil lubrication is known as means for keeping the rolling bearing lubricated and includes, for example, oil air lubrication, oil mist lubrication, and oil jet lubrication. However, these lubrication systems disadvantageously consume a very large amount of oil and thus increase running costs.
Thus, a bearing apparatus has been proposed in which a lubrication unit with a tank and a pump is assembled between an inner ring and an outer ring of a rolling bearing (see Japanese Patent Application Publication No. 2004-108388 (JP 2004-108388 A)). In the bearing apparatus, the lubrication unit is installed inside an axial extension portion of the outer ring of the rolling bearing. The rolling bearing (bearing portion) and the lubrication unit are integrated together. The lubrication unit includes the tank and the pump. The tank stores lubricant. The pump discharges the lubricant in the tank into an annular space between the inner ring and the outer ring. The pump has a needle-like nozzle protruding from a pump body. The pump discharges an approximately several microliter to several tens of nanoliter oil droplet through a tip of the nozzle. The oil droplet is fed to areas such as an inner-ring raceway surface and an outer-ring raceway surface to which the lubricant need to be supplied.
In the bearing apparatus with the lubrication unit assembled therein as described above, driving of the pump allows lubricant 91 to seep through an opening 90a at a nozzle tip 90 as depicted in FIG. 7A. When the amount of the seeping lubricant 91 increases, the lubricant is held at the nozzle tip 90 in the form of an approximately several microliter to several tens of nanoliter oil droplet as depicted in FIG. 7B. In the lubrication unit described in JP 2004-108388 A, the nozzle tip 90 is in proximity to balls 92 that are rolling elements. The oil droplet 91 held at the nozzle tip 90 are fed to a bearing interior through air flows occurring in the annular space between the inner ring and the outer ring as a result of rotation of the bearing.
However, in this case, the oil droplet 91 having left the nozzle tip 90 does not always reach the raceway surfaces, to which the lubricant needs to be supplied. For example, the oil droplet 91 may adhere to an outer surface of a cage that holds the rolling elements and fail to contribute to lubricating the rolling bearing. Thus, more lubricant (oil droplets) than needed may be discharged using the pump in order to provide for loss of the lubricant. However, the excess lubricant, which fails to contribute to lubrication, may increase rotating resistance (stirring resistance) to the rolling bearing.
In the bearing apparatus described in JP 2004-108388 A, the lubrication unit needs to be arranged in a space with a small volume, making an increase in the volume of the tank difficult. Thus, when the bearing apparatus is set to allow the pump to discharge a large amount of lubricant, consumption of the lubricant in the tank is increased, leading to the need to frequently refill the tank with lubricant. When the refilling with lubricant is frequently performed for maintenance, machine tools are shut down for each refilling, resulting in reduced production efficiency.